<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button>1</button>
    <button>2</button>
    <button>3</button>
    <button>4</button>
    <button>5</button>
    <script>
        var bts = document.querySelectorAll('button')
        for(var i = 0;i<bts.length;i++){
            bts[i].onclick = function(){
                for(var i=0;i<bts.length;i++){
                   bts[i].style.backgroundColor = ''
                }
                this.style.backgroundColor = 'pink'
            }
        }
    </script>
</body>
</html>